Upon a droplet adhering to a surface of a solid object, corrosion, deterioration, and pollution may expand from the adhered point. For example, an exhaust pipe of an engine is continuously exposed to high temperature air. Meanwhile it is known that rusting is occurred from a point at which a water droplet is adhered if the water droplet is adhered or condensation is occurred when temperature of the exhaust pipe is low. Further, if oil adhered to a solid subject is continued to be exposed to high temperature air, the oil is carbonized and adheres with a high strength. Therefore, visibility may be decreased and choke may occur. Therefore, in various fields of engineering, material and surface treatment is tried to be developed, which makes it possible to prevent deterioration of capability for removing droplets over a long period of time even in a high-temperature environment.
Recently, importance is placed on dynamic motion of the droplet (dynamic wettability) on the surface of the solid object as an index of capability of removing droplets, which can be evaluated by using contact angle hysteresis (e.g. Patent Document 1 and Non-Patent Document 1). The contact angle hysteresis is indicated by a difference (θA−θR) between an advance contact angle (θA) and a sweepback contact angle (θR), and the droplet slips on the surface of he solid object with a slight tilt as the value of the contact angle hysteresis becomes smaller. That is, the surface of the solid object whose contact angle hysteresis is small has an excellent capability for removing the droplets. On the other hand, the droplet is pinned at the surface of the solid object whose contact angle hysteresis is great even if the surface is super-water-repellent whose static contact angle exceeding 150°.
In non-patent documents 2 and 3, it is disclosed that a mixed solution of decyltriethoxysilane and tetramethoxysilane is coated on respective substrate, and a transparent film can be obtained when the solution is dried under normal temperature.